We measured respiratory flow, volume by integrated flow, and airway pr
essure in four mechanically ventilated infants. Compliance of the resp
iratory system (Crs) was measured by the multiple occlusion method. Ai
rway occlusion and release were achieved by a balloon in an occlusion
valve between the endotracheal tube and a pneumotachometer, all connec
ted in series with the respirator circuit. Values of Crs varied greatl
y due to changes in mean airway pressure (MAP). The Crs increased with
the elevation of MAP on some occasions and decreased on others. Value
s of Crs also varied corresponding to different occlusion pressures fo
r individual levels of MAP. Thus Crs changed continuously, even within
a single respiratory cycle. The observed variability in Crs was expla
ined by the following mechanism: a pressure-volume (P-V) loop of tidal
ventilation moves its position within the pressure-volume diagram and
changes shape due to variations of the ventilator settings. In some i
nfants the lungs were ventilated in the range of the linear portion of
the P-V diagram while in others they may have been ventilated on the
flattened portion. Some patients had convex expiratory P-V curves even
with low pressure swings during tidal ventilation.